In machines that are used in the production and testing of semiconductor components, for example, it is often necessary to position objects very accurately. It may be necessary, for instance, to position wafers underneath a tool, an exposure unit, or an inspection unit with the utmost precision. The wafer lies on a table that can be moved with six degrees of freedom via a corresponding drive. In other words, it is the table that is used as object whose position in relation to a reference system is to be recorded with the highest precision, while a stationary reference frame of the machine is used as reference system. To position the table via the drive and an associated control unit, it is necessary to generate position signals regarding the spatial position of the table in relation to the reference frame by high-precision position measuring devices.
Machines of this kind predominantly use interferometers as high-precision position measuring devices, but may also use grating-based optical position measuring devices. If grating-based optical position measuring devices are used, it is possible, for example, to place a single or multiple scanning unit(s) on the movable table, while a suitable measuring standard is situated on the reference frame. If this requires a position detection along two orthogonal main movement axes of the table, then the required measuring standard is arranged as a two-dimensional measuring standard in the form of a so-called cross grating. Problems arise if the table is moved across a relatively large range of traverse along one of the main movement axes, since correspondingly large, two-dimensional measuring standards are required in such a case. However, with increasing size, their production becomes much more complicated.
To avoid the production of very large two-dimensional measuring standards, it is conventional to use multiple individual two-dimensional measuring standards or cross grating plates, in order to thereby provide a larger measuring range. The individual two-dimensional measuring standards are then placed next to each other on the reference frame, along the particular main movement axis/axes. A corresponding system is described in U.S. Pat. No. 7,602,489, for example, which is expressly incorporated herein in its entirety by reference thereto. A total of four cross grating plates or two-dimensional measuring standards is mounted on the reference frame in a square arrangement, in order to cover the measuring range. For the optical scanning, a total of four scanning units is provided on the side of the object that is movable along two orthogonal main movement axes. The scanning gratings of the four scanning units are rotated at +/−45° in relation to the two main movement axes, the four scanning units more or less being situated in the corners of a square. The extension of this square corresponds to about the spatial extension of a single two-dimensional measuring standard. When the machine is in operation, defined switchovers take place between the different scanning units in order to ensure that no positional information will be lost when traveling across transition points between adjacent measuring standards. In order to also make it possible to detect the orientation of the table in addition to determining the position of the table along the two main movement axes, position values of at least three scanning units having two axes in each case must always be available simultaneously. The maximum range of traverse along a main movement axis in such a system corresponds to the overall extension of the adjacently situated cross grating plates along this main movement axis, minus the distance of the scanning units along this main movement axis.
In the system described in U.S. Pat. No. 7,602,489, a further increase in the range of traverse of the movable object along at least one of the two main movement axes would require further two-dimensional measuring standards or cross grating plates to be positioned along this main movement axis, or else these cross grating plates would have to have larger dimensions. However, both variants would require a much greater outlay for the overall system.